Certain types of network traffic in certain applications need to be inspected by a device which has lots of flexibility; e.g. a processor. The primary example of this is security, whereby every flow coming through a secured point (firewall) must be inspected to some greater or lesser degree. Hardware-based systems tend to be very fast, but don't deal well with very complex operations. Hence, software-based (processor-based) systems are still the norm, even with all of their concomitant performance problems. Fore instance, see U.S. Pat. No. 5,699,513, incorporated by reference herein.
Even in a system where a single processor is fast enough, if that processor dies, then the whole system grinds to a standstill. This is highly undesirable in a mission critical application (or in any application for that matter).